The light dependent membrane current of the hyperpolarizing photoreceptors in the scallop retina was measured at different voltages, under different external ionic conditions and at different light intensities. The light dependent potassium current is voltage dependent. The potassium conductance increases e-fold for about a 25 mV change in potential. The results suggest that some element in the channel responds to depolarization. The result of the substitution of different external ions and small molecules show that the light dependent potassium channel is very selective and resembles the voltage dependent potassium channel. The selectivity sequence for the light dependent channel is K ion, Tl ion greater than Rb ion, greater than NH4 greater than Cs ion greater than Na ion, Li. Organic molecules, other than hydrazine do not pass through the channel which is estimated to be about 3.3 A in diameter. The prolonged after hyperpolarization produced by intense short wavelength light was measured under voltage clamp conditions as well as the abolition of this current by long wavelength light. This effect occurs at all membrane voltages, depends only on the movement of potassium through the channel and is not abolished by the complete removal of external calcium ions.